1. Field
Embodiments of the present invention relate to a proximity sensor, and in particular to a proximity sensor for use in semiconductor lithographic applications.
2. Background
Many automated manufacturing processes require the sensing of the distance between a manufacturing tool and the product or material surface being worked upon. In some situations, such as semiconductor lithography, that distance must be measured with an accuracy approaching a nanometer.
The challenges associated with creating a proximity sensor of such accuracy are significant, particularly in the context of photolithography systems. In the photolithography context, in addition to the need to be non-intrusive as well as to precisely detect very small distances, the proximity sensor cannot introduce contaminants or come in contact with the work surface, typically a semiconductor wafer. Occurrence of either situation may significantly degrade or ruin the quality of the material surface or product being worked upon.
Different types of proximity sensors are available to measure very small distances. Examples of proximity sensors include capacitance gauges and optical gauges. These proximity sensors have serious shortcomings when used in lithographic projection systems because the physical properties of materials deposited on wafers may impact the precision of these sensors. For example, capacitance gauges, being dependent on the concentration of electric charges, can yield spurious proximity readings in locations where one type of material (e.g., metal) is concentrated. More generally, optical and capacitive methods are prone to errors due to significant interactions with layers beneath photoresist coatings. Another class of problems occurs when exotic wafers made of non-conductive and/or photosensitive materials, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP), are used. In these cases, capacitance gauges and optical gauges may provide spurious results, and are therefore not optimal.
U.S. Pat. Nos. 4,953,388, 4,550,592, 7,010,958, and 7,549,321, all of which are incorporated herein by reference in their entireties, disclose an alternative approach to proximity sensing through the use of a fluid sensor. In this application, the use of the word “fluid” includes the use of either liquid or gas forms of a substance. A typical fluid sensor contains a reference nozzle and one or more measurement nozzles to emit a fluid flow onto reference and measurement surfaces. Measurements are made of the back pressure differences within the sensors to determine the distance between the measurement nozzle and the measurement surface. A fluid sensor is not vulnerable to concentrations of electric charges or to the electrical, optical or other physical properties of a wafer surface. A fluid sensor detects only the top physical layer, and thereby yields a superior result. Accordingly, these types of gauges are ideal for topographic measurement of a material surface, such as that used to establish focus prior to lithographic exposure.
Speed of measurement is a critical performance driver in current semiconductor manufacturing processes. Although adding multiple measurement nozzles to a proximity sensor increases its throughput, it carries a disadvantage in that such an addition adds complexity and cost. Moreover, high bandwidth of proximity sensors is also a critical requirement to support current semiconductor manufacturing practice.
In addition to speed and bandwidth as being some of the key requirements, proximity sensors typically operate at very small gaps between the wafers and the sensors. As such, these sensors are often attached to an extend-and-retract mechanism, the stability of which affects the error budget of the sensor measurement. Faced with an error budget challenge of the type described above, the conventional practice for this class of proximity sensor is to use a balanced bridge architecture to gain common mode rejection of these types of external environmental disturbances. Use of a balanced bridge adds to the cost and the complexity of the sensor.
Therefore, what is needed is an apparatus and method to provide an accurate proximity sensor with increased measurement speed possibilities while minimizing the cost and complexity of the sensor.